A mechanical seal is a device configured to provide a sealing interface between a static housing and rotating shaft of a device, such as a pump, mixer or the like, for the purpose of inhibiting fluid within the device from escaping and/or external contaminants from entering the device. Mechanical seals are employed in a wide variety of industrial applications, processing media and operating conditions, where a gap between a rotating shaft and a static housing has to be sealed.
Referring to FIG. 1, a cross-sectional view of a mechanical seal 100 of the prior art is depicted. In this depiction, the mechanical seal 100 is configured to inhibit the flow of fluid and contaminants through a gap 102 between a stationary housing 104 and a rotating shaft 106. The mechanical seal 100 is generally comprised of an annular stationary ring 108 (also known as a primary ring) and annular rotating ring 110 (also known as a mating ring), a pair of seals or glands 112, 114 (which are generally, but certainly not limited to, elastomeric sealing elements such as O-rings), and a biasing member 116. While the mechanical seal 100 depicted in FIG. 1 comprises a single pair of sealing rings 108 and 110, various mechanical seals known in the art can comprise additional sealing interfaces, such as for example the double seal embodiments disclosed in U.S. Pat. No. 8,857,818 (which is assigned to the Applicant of the present application), the contents of which are incorporated by reference herein.
In operation, the annular stationary ring 108 remains fixed in position relative to the housing 104. Gland 112 is positioned between the annular stationary ring 108 and the housing 104 to inhibit the flow of fluid between these components. Annular rotating ring 110 rotates with the rotating shaft 106. Gland 114 is positioned between the annular rotating ring 110 and the rotating shaft 106 to inhibit the flow of fluid between these components.
Both the annular stationary ring 108 and the annular rotating ring 110 include smooth, contacting seal faces 109, 111, thereby forming a sliding seal interface 118. Accordingly, use of the mechanical seal 100 enables the radial gap 102—where fluid would normally escape—to be sealed by a flat, sliding seal interface 118 that is perpendicular to the rotating shaft 106, and therefore much easier to seal.
A biasing member 116, such as one or more coil springs and/or a bellows arrangement, can be positioned between a boss 120 on the rotating shaft 106 and the gland 114 and/or annular rotating ring 110 to urge the floating annular rotating ring 110 towards the annular stationary ring 108. In this manner, the biasing member 116 aids in maintaining contact between contact seal faces 109, 111 by accommodating small shaft deflections, shaft movement due to bearing tolerances and out of perpendicular alignment due to manufacturing tolerances.
Because annular rotating ring 110 rotates relative to annular stationary ring 108, there is naturally some wear on the seal faces 109, 111 during operation. In particular, wear of the sliding seal interface 118 can be accelerated in the presence of friction and heat generation. Excessive wear of the seal faces 109, 111 ultimately leads to failure of the mechanical seal 100.
To slow the rate of wear, often a lubricant, commonly referred to as a lubricating fluid or barrier fluid, is introduced into the seal interface 118. The lubricating fluid can be the fluid to be sealed, or it can be another barrier fluid introduced into the seal interface 118. In another example, the seal interface can be lubricated by a dry gas, such as a vapor of the sealed product, air or nitrogen. Maintaining the proper film thickness and flow of the lubricant within the seal interface 118 is an important aspect in minimizing the wear of the seal faces 109, 111. Accordingly, the geometry of the seal faces 109, 111 and the width of gap 102 are precisely controlled in these types of mechanical seals, as they play an important role in determining the film thickness and flow of the lubricant.
More advanced mechanical seal systems can include multiple mechanical seals, such as a dual or double mechanical seal. Such mechanical seal systems can be provided with more than a single lubricating fluid. For example, in some double seal systems, the first mechanical seal is lubricated by a vapor of the sealed product, and the second mechanical seal is lubricated with another liquid or gas compatible with the sealed product. In some cases, the lubricating fluid of the second mechanical seal can be maintained at a higher pressure to further minimize leaking of the sealed product to the atmosphere.
Mechanical seal systems, therefore, can include not only the mechanical seals themselves, but also seal support systems such as an external reservoir, a bladder or piston accumulator for liquid lubricated seals, and a gas treatment unit (GTU) for a gas conditioning unit (GCU) for gas lubricated seals. These units can include components that provide appropriate filtration, flow management, heating, cooling, and other conditioning of the lubricating fluids. Mechanical seal systems can also include piping, tubing, and other connective units needed to appropriately manage fluid flow across the seal, as well as the housing and/or the device that the mechanical seal is installed in relation to.
As with all mechanical systems, eventually the annular stationary ring 108 and the annual rotating ring 110 will wear out and need to be replaced. In some cases, the components of the mechanical seal 100 will simply reach the end of their useful life. In other cases, certain conditions will hasten wear on the components within the mechanical seal 100. Some of these conditions include misinstallation of seal components or improper seal selection, the seal faces opening during operation as a result of axial misalignment or improper loading, flashing (liquid to vapor transition that causes pulsating leakage and chatter of the seal), cavitation, or environmental conditions which can lead to a collapse of the thin film of lubricant.
Efficient operation and maintenance of rotating equipment is essential to maximize production capacity and minimize downtime. Moreover, unexpected catastrophic equipment failure can result in injury to personnel. Fortunately, in many cases the mechanical seal system will begin to show signs of distress in advance of a catastrophic failure, and in some cases indicate the remaining useful life of the components.
Conventional equipment monitoring is most often affected by a person who periodically visits the equipment, to make observations of noise and leaks, and take vibration readings with an accelerometer. The gathered information can then be compared with the historical data on the equipment to detect trends to indicate the overall health of the mechanical seal 100. Various methods for condition monitoring and diagnostics are discussed in International Standards Organization (ISO) 17359:2011, CONDITION MONITORING AND DIAGNOSTICS OF MACHINES—GENERAL GUIDELINES, and ISO 13381-1:2015, CONDITION MONITORING AND DIAGNOSTICS OF MACHINES—PROGNOSTICS, the contents of which are incorporated by reference herein.
One problem with this procedure is the time and labor costs involved. Another problem is the fact that the equipment is not constantly monitored, thereby enabling acute conditions, such as flashing, cavitation, and the negative effects of certain environmental conditions to occur without warning.
More advanced monitoring systems may employ one or more sensors that enable monitoring of the equipment and mechanical seal 100 in real-time. These sensors can include, for example, temperature sensors, pressure transducers, and accelerometers. Such sensors can be intrusive, requiring permanent or temporary insertion of a probe or sensor within the stationary housing 104, or they can be non-intrusive and capable of detecting sensed data from the exterior of stationary housing 104, or other components of the mechanical seal system. Such systems are particularly useful in applications where the equipment to be monitored is in a hazardous location or access to such equipment is generally impeded. Examples of such systems are disclosed in U.S. Pat. Nos. 6,082,737 and 6,325,377; and U.S. Patent Publ. Nos. 2013/0275056 and 2014/0161587 (all assigned to the Applicant of the present application), the contents of which are incorporated by reference herein.
Other systems, such as those disclosed in, for example, U.S. Pat. Nos. 8,651,801 and 9,145,783 (assigned to the Applicant of the present application), the contents of which are incorporated by reference herein, can further provide monitoring of the device, such as a pump, mixer or the like, that the mechanical seal is installed in relation to. Such advanced monitoring systems can provide limited amounts of control of the mechanical seal, seal support system or other components, in response to the monitored conditions. For example, the monitoring system includes a control algorithm configured to automatically mitigate the effects of a mechanical seal malfunction by adjusting certain operating parameters of the mechanical seal system.
In some cases, the various sensors of the monitoring system are installed to aid in identifying one or more previously identified ways in which the mechanical seal system may fail. The various ways in which a mechanical seal system may fail may be determined through a process referred to as Failure Modes and Effects Analysis (FMEA). FMEA is a step-by-step approach for identifying all of the possible failures in a mechanical seal system design. The term “failure modes” refers to the ways, or modes, in which the mechanical seal system might fail, a failure meaning any type of error or defect which may adversely affect the performance and/or longevity of the mechanical seal system.
Once the possible failure modes in a mechanical seal system have been identified, the effects of the failure modes are analyzed through a process referred to as “effects analysis” in order to gain an understanding of the consequences of the identified failure modes. Based on the effects analysis, the failure modes are prioritized according to the severity of their consequences, how frequently they are likely to occur, and how easily they can be detected.
The overall purpose of the FMEA is to take actions to eliminate or reduce failures, beginning with the highest priority failure mode. Accordingly, FMEA is typically used during the design phase to insulate against anticipated failure modes; however, it may also be used during operation. FMEA is discussed in International Electrotechnical Commission (IEC) Standard 60812:2006: ANALYSIS TECHNIQUES FOR SYSTEM RELIABILITY—PROCEDURE FOR FAILURE MODE AND EFFECTS ANALYSIS (FMEA), the contents of which are incorporated by reference herein.
When FMEA of a particular mechanical seal system reveals a high priority failure mode, certain operating conditions can be monitored through an advanced monitoring system to aid in determining whether the failure mode is occurring, or is about to occur. In particular, an expert in the field of mechanical seal systems, such as a designer, engineer or technician, based on their experience, can set a particular threshold or limit for a given operating condition relating to the high priority failure mode. Thereafter, during operation, an operator can be alerted if the threshold or limit of the monitored condition is exceeded.
Unfortunately, the information provided by the individual sensors of an advanced monitoring system in isolation has in some cases proved insufficient to make conclusive determinations about the overall health of the mechanical seal system. For example, a particular operating condition exceeding a predefined threshold or limit may indicate that a particular failure mode is occurring, but based on other operating conditions, the exceeded threshold or limit may also be an indication of a wholly different type of failure or event. Complex mechanical seal systems are known to experience failures for multiple interrelated reasons. Accordingly, the advanced monitoring systems developed to date can require a human operator with a requisite level of knowledge of the monitored mechanical seal system in order to properly diagnose failures and the overall health of the mechanical seal system.
Human operators have the advantage of being able to reason in the abstract and potentially pull information from their experience, but a human operator's effectiveness can be hampered by their inability to digest large amounts of sensor data. For example, in some situations, relevant data may be inadvertently ignored by the human operator, resulting in an improper diagnosis. In other situations, a delay in making a decision about the health of the mechanical seal system may lead to a scenario in which it is no longer possible to avoid an impending failure, whereas had the decision been made earlier, corrective actions could have been taken to avoid the failure.
Moreover, the cost of installing and operating such mechanical seal systems can be prohibitive. First, the monitoring of conditions for high priority failure modes may require a customized advanced monitoring system designed specifically for the mechanical seal system to be monitored. This is particularly true when the equipment to be monitored is unique or nonstandard, or where there are specific environmental conditions that require the system to be tailored to a particular application. Second, the operator of the advanced monitoring system must have the requisite knowledge in order to properly diagnose potential failures, which typically requires higher wages.
Accordingly, what is needed in the industry is a system and method that enables tailored customized advanced monitoring mechanical seal systems to be constructed and autonomously operated with improved reliability and increased speed, thereby alleviating the need for the mechanical seal system to be constantly monitored by a human operator during operation.